1. Field
The present specification generally relates to porous ceramic honeycomb articles and, more specifically, to cordierite porous ceramic honeycomb articles for use as filter materials.
2. Technical Background
Wall-flow diesel particulate filters (DPFs), gasoline particulate filters (GPFs) and ceramic catalyst supports generally comprise thin-walled ceramic honeycomb articles with high geometric surface areas and, in some cases, extensive interconnected porosity to facilitate fluid filtration. Ceramic filters, in particular, must exhibit high mechanical strength to facilitate handling and superior thermal shock resistance in operation to prevent degradation of the filter.
Cordierite honeycombs employed in exhaust gas after-treatment applications generally have low coefficients of thermal expansion and low Young's elastic modulus as a result of microcracks in the ceramic phase in the as-fired condition, attributes which are beneficial to thermal shock resistance. However, when such highly microcracked ceramics are washcoated with a catalyst washcoat, the catalyst washcoat penetrates into the microcracks, which can cause an increase in the CTE and elastic modulus of the article as the catalyst washcoat in the microcracks prevents the microcracks from closing during temperature excursions. Furthermore, the acidic solution typically employed in commercial washcoating systems can result in extension of the lengths of the pre-existing microcracks by stress corrosion, causing a reduction in the strength of the ceramic.
One solution to this problem has been to create a temporary “passivation” coating the microcracks as a barrier to the penetration of the catalyst washcoat into the microcracks during the coating process. This passivation step adds cost due to additional equipment, expended chemicals, and processing time, but is necessary to protect the thermal shock performance of the product. Another approach which has been taken to eliminate the problem of degradation in the thermophysical properties of cordierite honeycomb ceramics during washcoating has been the elimination of microcracks from the as-fired ceramic matrix (see references). The increase in the coefficient of thermal expansion associated with the elimination of microcracks can be balanced by an increase in strain tolerance (MOR/E) at higher porosity. However, the development of small degrees of microcracking during use can result in a substantial decrease in strength and reduction in strain tolerance without a substantial decrease in CTE, thereby resulting in a decrease in thermal shock resistance.
Accordingly, a need exists for alternative porous ceramic honeycomb articles which are less susceptible to thermally induced cracking and alternative methods for manufacturing the same.